The present invention relates to a semiconductor device and a method of manufacturing the same. Specifically, the present invention relates to a semiconductor device in which gate insulating films with three different levels of thicknesses are formed, and a manufacturing method for the same. Specifically, the present invention relates to a semiconductor device having gate insulating films with three levels of thicknesses formed therein, and a method of manufacturing the same.
Recently, developed is a semiconductor device of a multi-functional consolidated type formed on one chip. In order to make use of multiple levels of control voltages in such a semiconductor device, it is desirable that gate insulating films of MOSFET (Metal Oxide Semiconductor Field Effect Transistor) be also formed to have multiple levels of thicknesses.
For example, Japanese Patent Application Publication No. 2003-60074 discloses a technique to form gate insulating films having three different thicknesses in one chip. However, this conventional technique has the following problems. Specifically, because a silicon nitride film is used as a mask in the formation of these gate insulating films, it is hard to control a selective etching ratio between the silicon nitride film as the mask and a silicon oxide film serving as the gate insulating film. Accordingly, the shape of the gate insulating film is controlled to a lesser extent. Furthermore, the need to remove the silicon nitride film having been formed and used as the mask leads to an increase in the number of manufacturing steps.